I am using a DVI to VGA converter on my Ubuntu 12.04 machine to connect the graphics card (which doesn't have native VGA) to a Dell 2405 monitor on VGA. The Dell 2405 can handle resolutions up to 1920x1200 however the use of the VGA adapter has prevented Ubuntu from correctly reading the EDID data that the monitor provides, to detect the resolutions the monitor is capable of. At current I am stuck with the highest resolution of 1024x768. Xorg.conf has typically been the place in the past to address these issues however it seems to be becoming depreciated. Here's how I manually added the higher resolutions...

First off manually add and test the resolution you want. Note this will not be persistent after rebooting, you will lose the new resolution. Once you're happy the resolution works, you can then make it persistent between reboots.

You will cvt or gtf for generating the configurations for the monitor modes I believe cvt is standard with Ubuntu 12.04.

2. Create the new mode with xrandr - Take the modeline output from above and append it tot eh xrandr --newmode command. Note I had to drop the refresh rate "_60.00" from the new mode name for this to work. Without dropping the refresh rate the command refuse to work.xrandr --newmode "1920x1080" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync